Vacuum tube



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HTTOIZIUEY Patented Apr. 22, 1952 OFFICE VACUUM TUBE Samuel Freedman, San Diego, Calif., and Giusto Fonda Bonardi, New York, N. Y,

Original application June 13, 1945, Serial No.

1947, Serial No. 746,774

1 Claim.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention refers to vacuum tubes and a method of operation, and more particularly to such equipment for use with microwave or ultrahigh frequencies especially those of 300 megacycles per second and above.

An object of this invention is to provide a vacuum tube for detection and amplification together with a method of operation wherein the electrons flowing between the cathode and plate electrodes occur in groups representative of the frequency involved and wherein the elements may be spaced at considerably greater distance with respect to the frequency involved than can those of the special tubes heretofore used under similar conditions, and also wherein the phase relationship of the electron group becomes important and adjustable for maximum output efficiency.

Another object of this invention is to provide a vacuum tube of suitable design for microwavefrequency operation.

Another object of this invention is to provide a vacuum tube of coaxial design for microwavefrequency operation.

Other objects and advantages of this invention, as well as methods, systems, arrangement, construction and operation, will be apparent from the following description and claims in connection with the accompanying drawings, in which:

Fig. 1 indicates the grid versus plate characteristic of a conventional vacuum tube operating in normal manner in class B,

Fig. 2 is similar to Fig. l but indicates the response characteristic when grouping of the electrons occurs as referred to in this invention,

Fig. 3 indicates conflicting voltages in the plate and grid electrodes of conventional vacuum tubes when used at microwave frequencies,

Fig. 4 indicates an improved design for vacuum tubes for microwave frequency operation,

Fig. 5 indicates a conventional representation of electron grouping within the tube when the period of oscillation is shorter than the transit Divided and this application May 8,

2 application Serial No. 599,161, filed June 13, 1945, of which this application is a division.

Fig. 1 indicates the characteristic ID of plate output of a conventional vacuum tube operating in normal manner with the grid biased close to the cut-off point as occurs in class B operation, as contrasted with the straight line plate output characteristics II and I2 which occur as indicated in Fig. 2 when the electrons flow within the vacuum tube in groups.

Grouping of the electrons, as fully described in copending applications Serial No. 587,544, filed April 10, 19%, and Serial No. 595,298, filed May 23, 1945, and now abandoned, may be graphically illustrated as shown in Fig. 5 wherein, upon application of microwave frequency potentials to the grid [3, the electrons leave the cathode l4 and form into groups l5, one group for each cycle, and are constantly accelerated by the plate potential until they reach the plate l6. It will thus be seen that the phase relationship of the roups with respect to their arrival at the plate should be correct for maximum output power. The phase relationship may be controlled by altering the plate potential and hence the speed of acceleration.

In the case of a vacuum tube used as a detector, when the grid of a conventional triode is biased near the cutoff point and a high frequency alternating potential is applied to the grid member, only the positive peaks of the applied potential will allow current to flow to the plate. If the plate circuit is not tuned and a bypass is provided in the plate circuit for the high frequency ripple, then the current flowing in the plate circuit will reproduce the average value of the peaks of the current flowing from cathode to plate within the tube. In case the applied high frequency voltage is amplitude modulated, modulation will appear in the output of the tube.

Amplification takes place also according to the characteristics of the tube and the plate circuit, and is roughly proportional to the transconductance of the tube.

Transconductance is the ratio Ip/Vg where Ip is the alternating component of the plate current and Vg is the alternating component of the grid voltage, the ratio being more accurately defined by use of derivatives. It is equal to the slope of the curve of plate current versus grid voltage for a stated plate voltage. This curve, called the tube characteristic, is indicated in Fig. l as I0 for a conventional triode.

When the frequency of the signal is very high, the tube shows its conventional transconductance only for the direct-current component of the plate current while the alternating component is ruled by the dynamic transconductance, as explained in the copending applications hereinbefore mentioned. The dynamic transconductance begins to taken on importance only when electron grouping takes place within the tube, and increases in value and importance as the frequency is increased. The dynamic transconductance is represented in Fig. 2 for one value of voltages and frequency where the tube characteristic II is indicated for lower, and the characteristic l2 for higher, frequencies.

If the tube functions as a detector, and the frequency of the input signal is high enough to result in dynamic transconductance, the output of the tube will be determined by the rectified peaks of plate current which are proportional to the dynamic transconductance of the tube. Then, if the dynamic transconductance is higher than the conventional transconductance the performance of the tube will be higher and will increase with increasing frequency as indicated in 'Fig. 2 by tube characteristic H and I 2.

While the shunting eifect of the grid-cathode capacitance increases with the first power of the frequency, the theory, previously referred to, shows that the dynamic transconductance increases with the square of the frequency so that the overall performance of the tube will be better at higher. frequencies. A limit is found only in the physical dimensions of the electrodes which may be longer than a quarter wavelength, resulting in partial phase shift of currents and voltages along the electrodes as they are considered a part of the coaxial line in case of concentric electrodes as indicated in Fig. 3, where I1 is a vacuum tube containing cathode l8, grid l9 and plate with cathode, plate and grid terminals indicated respectively as C, P and G.

The schematic design of a vacuum tube to overcome the difficulties just mentioned is indicated in Fig. 4 where short electrodes, though not located close together, reduce also the interelectrode capacitance eifect. The plate may be reduced to a simple ring 2| and the grid to a short row of :parallel wires or rings 22 surrounding the cathode 23. Beam confining plates 24 may also be used to confine the electrons 25 to the area required.

Another schematic design for a microwave tube is shown in Fig. 6. Here the electrodes are actually designed as substantial parts of coaxial lines or cables. This arrangement permits making the overall length of the electrodes equal to several wave lengths, thereby increasing the power output of the tube and the heat dissipation ability of the plate.

Fig. 6 represents a screen grid tube without enclosing envelope provided with control grid 26, screen grid 21, cathode 28, and anode or plate 29. The cathode may be heated by means of energy supplied by leads 30. As indicated, the various elements form coaxial lines with energy flowing within the areas indicated by the legends Input and Output.

In Fig. 7, curve 3| indicates the voltage between the electrodes, corresponding to microwaves of less length than the overall length of the electrodes. Under these conditions a series of groups of electrons, representative of the maximum and minimum points of the curve 3|, will occur longitudinally with respect to the electrodes nd between them. It is also understood that additional grids-and other electrodes may be provided as desired or required for any particular purpose and that, if desired, the anode or plate 29 may be utilized also as an enclosing envelope.

It will be understood that various modifications and changes may be made in this invention Without departing from the spirit and scope thereof as set forth in the appended claim.

What is claimed is:

An ultra-high-frequency electron discharge tube comprising an evacuated envelope, an elongated cathode extending axially of said envelope and energizable to provide a beam Of-BIBOtIOIlS, a grid surrounding said cathode and adapted for energization at ultra-high frequencies, an annular anode surrounding said grid, the axial dimension of said anode being only a small fraction of those of said cathode and grid, and beam deflection means disposed at the opposite ends of said grid for deflecting the electrons toward said anode.

SAMUEL. FREEDMAN. GIUSTO- FONDA BONARDI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Great Britain Mar. 18, 1926 

